A known injector is provided with an injection valve having a valve seat, which ends in an injection nozzle and is coupled with a pin capable of moving between a closed position of the valve seat and an open position of the valve seat under the thrust of an actuator; typically the actuator comprises a spring capable of keeping the pin in the closed position and an electromagnet capable of moving the pin from the closed position to the open position against the action of the spring.
Injectors of the type described above are usually called injectors with electromagnetic pin actuation and are very widespread because they combine good performance and low cost. However, the market demands injectors with better dynamic performance and capable of operating at very high pressures of diesel oil; for this reason, injectors have been proposed with hydraulic pin actuation, that is injectors in which the displacement of the pin from the closed position to the open position against the action of the spring happens by the effect of forces of hydraulic origin.
An example of an injector with hydraulic pin actuation is provided by patent application EP-1036932-A2 (or EP-0921302-A2), in which a lower portion of the pin is housed in an injection chamber, which is delimited at the bottom by the valve seat for the injection valve and an upper portion of the pin is housed in a control chamber, which houses the spring that holds the pin in the closed position; diesel oil is fed constantly at pressure either to the injection chamber, which it leaves via the injection nozzle when the pin is in the open position, or to the control chamber. The control chamber is coupled to a control valve, which is actuated by an electromagnetic actuator in order to move between a closed position and an open position, in which it puts the control chamber in communication with a low-pressure exhaust environment.
In use, when the control valve is closed, the pressure of the diesel oil in the control chamber is equal to the pressure of the diesel oil in the injection chamber, and the pin is held in the closed position either by the action of the spring, or by the hydraulic force that is generated because the area of the pin subjected to the action of the diesel oil is higher in the upper portion housed in the control chamber than in the lower portion housed in the injection chamber. When the control valve is opened, the pressure of the diesel oil in the control chamber tends to fall to much lower values than the pressure of the diesel oil in the injection chamber, and the pin is moved upwards in the open position by the effect of the hydraulic force that is generated by the difference in the pressures.
Another example of an injector with hydraulic pin actuation is provided by patent application WO-0129395-A1, in which an upper portion of the pin is housed in the control chamber, while a lower portion of the pin is housed in an injection chamber, which is delimited at the bottom by the valve seat of the injection valve and houses the spring that holds the pin in the closed position; the control chamber is coupled to the control valve, which is actuated by a piezoelectric actuator in order to move between a closed position and an open position in which it puts the control chamber in communication with a low-pressure exhaust environment.
A further example of an injector with hydraulic pin actuation is provided by patent U.S. Pat. No. 5,664,545-A1, which discloses a fuel injection apparatus including a casing having a control pressure chamber for storing fuel supplied from fuel passage, a needle valve to which fuel stored in the control pressure chamber applies pressure in the valve closing direction, a valve device for interrupting communication between the fuel passage and the control pressure chamber to seal fuel in said control pressure chamber, and volume changing device for expanding volume of the control pressure chamber after fuel is sealed in the control pressure chamber by the valve device; pressure in the control pressure chamber is reduced while the fuel is stored therein by the volume changing device, the nozzle needle is lifted, and injection is started.
It has been observed that in injectors with hydraulic pin actuation of the type described above, when closed the valve body of the control valve tends to rebound against the valve seat of the control valve causing a delay in the effective closing of the control valve and, therefore, of the injection nozzle; in this way, a random, variable error is introduced into the measuring out of the diesel oil, which random error has substantially little influence when operating with long injection times and, therefore, high amounts of injected fuel but is important when operating with short injection times and, therefore, low amounts of injected fuel. This disadvantage is particularly problematic in modern internal combustion engines with direct diesel injection, which, before the main injection of the diesel oil, carry out a series of pilot preinjections close together and marked by very short injection times.